By: Judit Monis, Ph.D
This year I attended a session organized by the Unified Wine and Grape Symposium on Sustainable Pest Management. The invited speakers were Marc Fuchs, professor at Cornell University, Steven Lindow, professor at the University of California-Berkeley, and Dr. Michelle Moyer, extension specialist at the University of Washington. Each of the speakers presented an update on their research which I summarize below with some of my own opinions.
Sustainable Control of Leafroll and Red Blotch Viruses
Marc Fuchs presented his research on leafroll and red blotch viruses. These viruses can cause revenue reductions as high as $3700/acre. Presently, the recommended management of these viruses is done by reducing the source of inoculum (i.e., rogue infected vines). The practice of rogueing or removal of infected vines from the vineyard reduces the amount of virus available to the insect vectors, therefore reducing the transmission rate. Economic studies indicate that removal of individual infected vines must be done if less than 25% of the vineyard block is infected. If there is more than a 25% infection, the recommendation is to remove the entire block.
Ideally, viral and other diseases would be controlled using disease resistance genes present in the plant. Unfortunately, no natural resistance to leafroll or red blotch viruses have been found in commercial or wild grape species. In a project funded by the California PD GWSS Board (PD Board) Professor Fuchs’ group has proposed to develop dual resistance to leafroll and mealybugs by modifying the genome of grapevine varieties using a gene silencing technology known as RNAi. Using genetic engineering it is possible to introduce portions of DNA that specifically will degrade the nucleic acid of the infecting virus and the mealybug able to transmit it. This method is being successfully used in Hawaii to control Papaya ringspot (PRSV), a virus that had decimated the papaya production in the islands. Thanks to this technology, Hawaii is able to produce healthy papayas that resist PRSV. Simply described, the plant genome would be able to detect viral and/or insect nucleic acids and chop them up before they are able to sequester the plant’s metabolic machinery to their advantage and cause disease.
Biological Control of Xylella fastidiosa the Causal Agent of Pierce’s Disease in Grapevines
Steve Lindow presented the research focused on the use of an endophytic bacteria capable of reducing the incidence of Xylella fastidiosa (Xf) in grapevines. Pierce’s Disease (PD) is a devastating disease and in the past years an increase in the infection rates of the bacteria has been seen in California vineyards. It is not clear what is the reason for the sudden increase of disease incidence. It is theorized that the increased infections could be due to the ability of other insects (cicadas, spittlebugs) to transmit the bacteria or perhaps due to climate change (less severe winters in California).
In a project funded by the PD Board, Professor Lindow’s group has discovered an endophytic bacterial species that appears to inhibit Xf propagation in grapevine and acts as a biological control agent. An endophyte is an organism capable of living and multiplying in its host (in this case grapevines) without causing disease. The endophytic bacteria is Paraburkholderia phytofirmans strain PsJN (Pf PsJN). As described at the presentation, the endophytic Pf PsJN acts as a vaccine activating the disease resistance pathways in the colonized plants. The endophytic Pf PsJN can multiply rapidly in the colonized grapevines without causing disease.
Furthermore, Pf PsJN is able to reduce the population of Xf and avoid disease development in experimental vines. The challenge presented in the application of this biological control agent (as well as others) is to develop an appropriate delivery system. Initial experiments to introduce Pf PsJN were done by prickling the leaves of the vine, however, this would not be practical in a commercial setting. So far, experiments using a surfactant to help the bacteria penetrate the grapevine’s conductive tissue (xylem) have been successful. The work is in progress and has shown that Pf PsJN is capable of inhibiting Xf disease development even after Xf infection had started. Research will continue and is focused on determining the timing of application that would offer the best disease protection. It is expected that once developed, the product will need to be tested and registered with the EPA as biological pesticide before its commercialization and/or use in the field.
Use of UVc Light (Germicidal Lamps) to Control Powdery Mildew
Dr. Michelle Moyer described the powdery mildew disease agent as a “Goldilocks fungi”, as conditions must be “just right” for infections to occur. The temperatures must not be too cold, and not too hot. The humidity cannot be too high or too low. It is well known that controlling powdery mildew in the vineyard is probably the largest pest control item line in the budget. We all know that there are management practices that can be applied to reduce the incidence of powdery mildew such as leaf thinning, opening the canopy to favor air flow and light but these are not sufficient to avoid the multiple powdery mildew season’s infections in a vineyard.
Over the years, in spite of the practice of rotating the different chemistries available for control, the powdery mildew fungi have mutated due to the selection pressure. Presently many resistant strains have evolved that are not affected by the different fungicides available, even when used in increased doses. The presentation by Dr. Moyer focused on work done by her group and others at Cornell University on the application of UVc light in the vineyard to control powdery mildew. A prototype machine was fabricated that is able to cover the canopy and emit UVc light capable of killing the fungus.
It is known that the powdery mildew pathogen is epiphytic and has adapted to resist day light UV lights. However, it has not adapted to UVc when applied at night. The UVc light is obtained from germicidal lamps, the same ones that are used to eliminate harmful human bacteria from tainted food or water. This technology is presently used to control powdery mildew in commercial strawberry fields in California and perhaps soon will be applied in the vineyard. The research continues and focuses on constructing the best equipment that is able to deliver the UV light in commercial vineyards.
The session presented alternative methods that can be applied for the control of different diseases. In a time, when insects and fungal pathogens are becoming resistant to chemicals it is refreshing to learn of alternative sustainable methods. These methods promise to have less impact on the environment as they are developed to target the pathogen and do not affect beneficial organisms.
I cannot emphasize the importance of planting material that has been tested free of important pathogens. However, once planted, vines are susceptible to become infected. As mentioned above, for virus control, the most efficient practice is to reduce the source of infection by eliminating diseased vines. In my experience, it is difficult to convince the grower to remove entire blocks. Sadly, often times, they will agree to remove and replant half of the block to avoid missing the production of grapes. In all cases, this practice is risky as invariably by the time the grower is ready to replace the portion of infected vines, the new plantings have become infected.
With no sources of resistance to important viruses available, pathogen derived genetic resistance is the only hope to keep vines from becoming re-infected in the vineyard. Presently, genetic modified technology is not widely accepted by the general public but we need to work hard to educate consumers that this is a sustainable method for disease control.
It is my hope to see more of these “out of the box” approaches on the control of grapevine diseases and pests will be used in the near future.
Judit Monis, Ph.D. provides specialized services to help growers, vineyard managers, and nursery personnel avoid the propagation and transmission of diseases caused by bacteria, fungi, and viruses in their vineyard blocks. Judit (based in California) is fluent in Spanish and is available to consult in all wine grape growing regions of the world. Due to the COVID19 pandemic, Judit is offering virtual vineyard visits. Contact email@example.com for more information or to request a consulting session. To learn more about the vineyard health services offered, please visit juditmonis.com